Women are between 2 and 10 times more likely to injure their anterior cruciate ligament (ACL) than men participating in similar military and athletic activities. Annual costs for surgical repair and rehabilitation of ACL injuries are over $850 million in females, alone. These expenses do not include the long-term costs related to the premature osteoarthritis (OA) that often follows the initial trauma. Gender specific differences in the hormonal milieu and response to specific steroid sex hormones have been suggested as potential causes for the disparities in ACL strength and injury risk between males and females. Most ACL injuries result from high tensile loads applied to the ligament during cutting and jumping. Type-I alpha-1 (T1C) and Type - 3 collagen (T3C) are the crucial structural components that define the ligament's ability to withstand tensile loads. If sex hormones influence ACL strength in a gender specific manner they must do so by mediating the mRNA transcription that regulates the T1C and T3C protein content essential to the ligament's mechanical strength. The mechanism and time course for sex hormones to modulate the collagen content and strength of the ACL is unknown. We hypothesize that steroid sex hormones mediate gender specific differences in the Type I and Type III collagen content and the mechanical strength of the ACL. To test this hypothesis the studies proposed in these specific aims will use a hydraulic materials testing device and Western blot analysis to measure ACL mechanical strength and collagen protein content in skeletally mature male and female rats injected with testosterone, estradiol or a combination of these hormones for 15 days. In our first specific aim the tensile strength of the ACL will be tested to demonstrate that sex hormones mediate gender specific changes in the strength of the ACL. Our second specific aim will clarify the mechanism by which sex hormones mediate changes in ligament strength by using Western blot analysis to demonstrate that selected steroid sex hormones mediate gender specific differences in T1C and T3C protein in the rat ACL. Accomplishing these aims will improve our understanding of the mechanism by which sex hormones influence collagen content and ACL strength and further explain the gender disparities in ACL injury rate. These findings are important as they will help direct future studies to identify women at risk for ACL injury and develop novel strategies to prevent their occurrence. This project is designed to determine the influence of selected steroid sex hormones on the collagen content and strength of the rat anterior cruciate ligament. It combines Western Blot analysis and materials testing in an in-vivo model. The proposed studies will improve our understanding of the mechanism by which sex hormones influence collagen content and ACL strength and further explain the gender disparities in ACL injury rate. These studies are important and they will help direct future studies to identify women at risk for ACL injury and develop novel strategies to prevent their occurrence.